1. Field of the Invention
The present invention relates to a radio-over-fiber link system for supporting various services.
2. Description of the Related Art
With the diversification and rapid increase of information and communication services, it becomes necessary to provide very high speed wireless multimedia communication service through combination of the optical communication technology and the wireless communication technology. Accordingly, interest is being focused on a technology of applying a microwave to a very high-speed optical communication network, by combining a wired communication technology with a wireless communication technology so as to enable various large-capacity multimedia information and communication services. Particularly, research is being actively conducted on a synthetic technology including two types of incorporated technologies, that is, on a radio-over-fiber (ROF) technology, which simultaneously uses an optical communication technology for high-speed transmission and a wireless technology for mobility.
Such an ROF technology basically uses an optical link apparatus and a radio link apparatus as basic components. The optical link apparatus modulates a transmission signal into a microwave-band signal, converts the microwave-band signal into an optical signal, and then transmits the optical signal through an optical fiber. The wireless link apparatus wirelessly carries a signal which has been received through the optical fiber. With respect to the ROF technology, researches are being actively conducted to develop a system capable of efficiently providing various wireless services for voice, broadcasting, data, etc., by tacking into consideration the demand for a broadband and the characteristics of the optical and wireless communications. In an environment in which various wireless services for voice, broadcasting, data, etc., are provided, it is inefficient to construct a remote antenna link every type of service. For this reason, the ROF link technology enables sharing of various types of service systems; so as to allow simultaneous transmission of multiple wireless services through one link; thereby improving the transmission efficiency.
Wireless communication systems use a frequency division duplexing (FDD) scheme which uses different frequencies in order to discriminate between uplink and downlink, and a time division duplexing (TDD) scheme which uses different transmission times in order to discriminate between uplink and downlink. An in-building solution or a common base station for wireless communication systems, which use different duplexing schemes (i.e., TDD and FDD schemes), requires an RF front-end device having a new structure.
An RF front-end device, which simultaneously supports TDD and FDD systems, can reduce the size of a base station or in-building system and can reduce the cost for system construction by sharing an amplifier and/or an antenna, when the TDD and FDD systems are used together in the base station or in-building system. In order to transmit a wireless signal to the RF front-end device as described above, the ROF technology is used.
FIG. 1 is a block diagram illustrating the construction of a central station (CS) 11 and a remote access unit (RAU) 12 in a conventional ROF link system for supporting various services, in which an ROF link structure including a front-end device capable of simultaneously supporting TDD and FDD systems is shown. Since the TDD system discriminates between uplink and downlink based on time bands and the FDD system discriminates between uplink and downlink based on frequency bands, it is difficult to separate a signal, to which TDD and FDD signals have been multiplexed, into uplink and downlink signals by using a switch or a filter. Therefore, FIG. 1 shows a case of employing a circulator in order to discriminate between uplink and downlink signals.
The CS 11 includes an electric-optical converter 111 and an optical-electric converter 112. The electric-optical converter 111 converts a downward signal, to which TDD and FDD signals have been multiplexed, into an optical signal, and transmits the optical signal to the RAU 12. The optical-electric converter 112 converts an upward optical signal, which has been transmitted through an upward optical fiber 14 from the RAU 12, into an electric signal (i.e., an RF signal). The RAU 12 includes an optical-electric converter 121, a downward high-power amplifier (HPA) 122, a circulator 123, an upward low-noise amplifier (LNA) 125, and an electric-optical converter 126. The optical-electric converter 121 converts a downward optical signal transmitted from the CS 11 into an electric signal, and the downward high-power amplifier 122 amplifies a downward signal output from the optical-electric converter 121. The circulator 123 allows a signal amplified by the downward high-power amplifier 122 to be emitted through an antenna, and establishes a signal path to provide a signal received through the antenna to the upward low-noise amplifier 125. The upward low-noise amplifier 125 amplifies a signal provided from the circulator 123, and the electric-optical converter 126 converts a signal output from the upward low-noise amplifier 125 into an optical signal and transmits the converted optical signal through the upward optical fiber 14 to the CS 11.
In FIG. 1, the circulator 123 outputs a downlink input signal, which has been received through a first port thereof, to a second port thereof, and outputs an uplink signal, which has been received through the second port thereof, to a third port thereof, thereby separating received signals into a downlink signal and an uplink signal. However, when an electric circulator is used as the circulator 123, a part of a downlink signal received through the first port thereof may be output to the third port thereof because the electric circulator has a low isolation. Also, when impedance matching is not completely achieved between the circulator and the antenna, a part of a downlink signal may reflect from the antenna, and be input to the second port of the circulator, thus the downlink signal may be output through the third port thereof. Generally, since large signal loss occurs in the air environment, the intensity of an uplink signal input through the antenna is very low. Therefore, the downlink signal output through the third port of the circulator has a relatively higher intensity than that of a received uplink signal, thereby saturating the amplifier for uplink, so that a downlink performance may deteriorate.
It is possible to reduce the intensity of an FDD downlink signal flowing into an uplink path by using a band stop filter. However, even in this case, it is impossible to reduce the intensity of a TDD downlink signal flowing into the uplink path, so that such a problem exerts a negative effect upon both uplinks of TDD and FDD systems.
Also, in the case of downlink in FIG. 1, TDD and FDD downlink signals are converted into optical signals by the electric-optical converter of the CS and are then transmitted through an optical fiber. The transmitted optical signals are converted into RF signals by the optical-electric converter of the RAU, and are output into the air via the amplifier, the circulator, and the antenna. In the case of uplink, an upward signal input into the antenna of the RAU passes through the circulator and the uplink amplifier, is converted into an optical signal by the electric-optical converter, and is then transmitted through the optical fiber. The transmitted signal is converted into an RF signal by the optical-electric converter of the CS. In such an ROF link, a non-linear phenomenon of the electric-optical converter exerts a large influence upon the entire system. A downlink signal flowing into an uplink path in an RF front-end device using a circulator may saturate the low-noise amplifier and/or the electric-optical converter or may cause operation of these devices with a lower current than an operational threshold current, thereby considerably deteriorating the uplink performance.
FIG. 2 is a block diagram illustrating the construction of a central station (CS) and a remote access unit (RAU) in another conventional ROF link system for supporting various services. According to the construction shown in FIG. 2, a CS 21 transmits upward/downward control information about upward/downward TDD signals to an RAU 22, and a switch 223 of the RAU 22 separates the upward/downward TDD signals from each other based on the control information, in order to prevent a TDD downward signal from exerting an influence upon an upward signal. In detail, the CS 21 of FIG. 2 has a construction similar to that of the CS 11 shown in FIG. 1, except that the electric-optical converter 211 also multiplexes a control signal for transmission of upward/downward control information about TDD signals together with TDD and FDD signals, and transmits the multiplexed signal through a downward optical fiber 23 to the RAU 22. The RAU 22 includes a demultiplexer 224, which separates the control signal from a downward signal received through an optical-electric converter 221, and provides the separated control signal to a controller 229. Also, the downward signal passes through the demultiplexer 224 and a downward amplifier 222, and is then input to a first duplexer 225. The first duplexer 225 separates the downward signal into a TDD downward signal and an FDD downward signal, and then provides the TDD downward signal to the switch 223 and provides the FDD downward signal to a triplexer 228. The triplexer 228 outputs the TDD downward signal provided through the switch 223 and the FDD downward signal provided from the first duplexer 225, to an antenna. Also, the triplexer 228 separates an upward signal input through the antenna into an FDD upward signal and a TDD upward signal, and then provides the TDD upward signal to the switch 223 and provides the FDD upward signal to a second duplexer 227. The second duplexer 227 combines the TDD upward signal provided from the switch 223 with the FDD upward signal provided from the triplexer 228, and provides the combined signal to an upward amplifier 225. In this case, the controller 229 controls a switching operation of the switch 223 based on the control signal such that the switch 223 either provides a TDD downward signal received from the first duplexer 225 to the triplexer 228, or provides a TDD upward signal received from the triplexer 228 to the second duplexer 227.
According to the conventional system as shown in FIG. 2, it is possible to prevent a TDD downward signal from exerting an influence upon an upward signal, but it has problems in that it is necessary to generate a separate control signal and to allocate a separate channel to transmit the control signal.